Over and over

Here's something I could never figure out - how can artists perform the same lines over and over and not lose their enthusiasm? I am specifically thinking of musicians and Broadway actors. It seems to me that the more you repeat something, the more tiresome it gets over time. At least as comedians we can move some jokes around even if we are performing the same act. But actors and musicians have to repeat themselves, word for word, every time. How do they do it?